docs: def reduce_slice(self, start, end, options=None):
          """
          Adds the Javascript built-in ``Riak.reduceSlice`` to the
          query as a reduce phase.
  
          :param start: the beginning of the slice
          :type start: integer
          :param end: the end of the slice
          :type end: integer
          :param options: phase options, containing 'language', 'keep'
            flag, and/or 'arg'.
          :type options: dict
          """
          if options is None:
              options = dict()
  
          options['arg'] = [start, end]
          return self.reduce("Riak.reduceSlice", options=options)